Among the most basic of needs of any mariner on the water, is the need to know the heading to the next waypoint. With the advancements in electronic navigation, it has become much easier to travel from waypoint to waypoint. That is, as long as your electronics are operating properly.
In a pinch, it always helps to know how to do things “the old fashioned way.” The old fashioned way involves the use of a magnetic compass. Our marine navigation charts provide us with a “compass rose.” The compass rose affords us the basic elements required to plot a course line between two points.
The compass rose supplies several things to the navigator. The outer part of the compass rose illustrates 360 “true” degrees, from which a “true” course can be plotted on the chart. The “inner” part of the compass rose illustrates 360 “magnetic” degrees, all of which include “variation.” Variation is the angular difference between “true” and “magnetic” north.
Variation is the result of a magnetic disturbance that causes a compass to point a few degrees “away” from true north. Variation can cause a compass to indicate a reading that is either east or west of true north. This phenomenon is measured and expressed in some number of degrees “east” or “west” of what otherwise would be a true reading.
What is known as “magnetic” north is not in the same geographic location as “true” north. That little complication causes the value of variation affecting your compass to change with the location of your vessel on the globe. Imagine that true north, magnetic north and your vessel are the three points of a triangle. The shape of such a triangle would be different for each location on the globe.
There are an infinite number of triangle shapes comprised of those three points. Therefore, there are an infinite number “orientations” of your vessel relative to “true” and “magnetic” north. Each unique orientation yields a different value for variation. The variation for each geographic area on the globe is the third thing that is provided in the compass rose, and is printed in the center of it.
The compass Rose story doesn’t end there. The magnetic disturbance that causes the variation between true and magnetic north is not static. That’s right, it is constantly moving. Printed in the center of each compass rose is the “annual increase” or annual decrease” that must be applied to the value given for variation. The movement below the earth’s crust includes the disturbance that results in “magnetic north” as well.
What does this all mean to you? It means that if your GPS fails to work, you might have to plot a course between two points on your chart. You can walk the plotted course line over to the compass rose. At the compass rose, you can determine both the true and magnetic course to your destination. The magnetic course can be used to steer your vessel to a destination according to a compass heading . . . except for one more thing.
There is another factor that causes your compass to be off by a few degrees. It is called deviation, and is the result of magnetic interference of equipment on your vessel. But that is a topic for another day!
Until next time, we wish you clear skies, fair winds and calm seas!
World Wide Marine Training, LLC, is a U.S. Coast Guard Approved facility authorized to give examinations for captain’s licenses up to Master 200 Tons, Able Seaman up to Unlimited, STCW Basic Training, Radar, ARPA and other Endorsements. Please visit www.worldwidemarinetraining.com or call toll-free 866-249-2135.